human brain

For a long time, mankind believed that the brain was a homogenous "blob", a black box with its processing ability spread throughout. This vague notion of "distributed computation" is perhaps what our first intuition suggests when faced with a piece of machinery of the complexity of the brain.

However, the psychologist and biologistRoger Sperry made many important early steps in creating the current model of the brain as a modular device, and indeed he won the 1981 Nobel Prize for his efforts. His work was inevitably based mainly on the brains of animals, since his primary technique was to sever various connections within the brain and then studing the effects. For example, working in conjuction with fellow biologist Ronald Myers, Sperry removed the corpus callosum - that part of the brain that connects to two hemispheres. He then taught one hemisphere a skill, namely pressing a lever, and left the other ignorant.

Of course, no such procedure could be carried out on humans. Luckily though, a number of people had had their corpus callosum split for medical reasons but were otherwise unimpaired and therefore suitable test subjects. I can only assume they were consenting in the experiment I am about to describe.

The Experiment

In my opinion, this is one of the cleverest experiments ever carried out in psycho-biology. It's results make fascinating reading.

The subject of the experiment was N.G., a Californian housewife. Remember that the two halves of her brain were completely unable to communicate since the "bridge" between them had been removed.

The woman was asked to view a white screen with a small black dot in the centre, and to focus her eyes on the dot. Sperry then flashed an image of a cup onto the right side of the screen for a very short period of time - approximately 1/20 of a second. While this is just long enough for the right eye to detect it, it is not long enough for both eyes to focus on the object. Therefore, the image is sent to the left hemisphere (recall that the left hemisphere controls the right side of the body and vice versa) but not the right. When asked what she had seen, N.G. replied "a cup". Nothing too surprising so far. (I guess this is some kind of control to check that N.G. could correctly identifty a cup...)

Now, of course, Sperry flashed the image onto the left side. The image now went exclusively to the right hemisphere. Sperry once again asked what her what she had seen. This time, N.G. replied, "Nothing". Finally, Sperry asked her to reach into a bag with her left hand and select, purely by touch, the item whose image she had just seen. The others items available included a knife, a pen and a comb. N.G. correctly selected the cup. Finally, still without seeing the object in her hand, N.G. was asked what she was holding. Response? "A pencil."

Most right-handed people have the centre of speech located in the left hemisphere of the brain - this was already known. Therefore, when the image was passed to the left side of the brain, as in the first case, N.G. was perfectly able to describe what she had seen. However, when it was passed to the right side, and she was again asked, the right side of the brain was unable to answer, having no speech module. Instead, the left side described what it had seen; that is, nothing. Usually this information would be passed across the corpus callosum, but as a "split-brain" patient, N.G. could make no such exchange. However, the left hand - controlled by the right side of the brain - could certainly be directed to pick out the correct item, which it did. However, when asked what she was holding, the same problem presented itself - the speech module in the left hemisphere was still altogether ignorant of the cup. This time, it took an educated guess - "a pencil", it said.

From this one experiment, we can learn a number of things about the way information is distributed between the two hemispheres, and confirm earlier beliefs about the position of the centre of speech. The experiment is not without its flaws (for the sake of completeness, Sperry should perhaps have performed the same experiment with a normal person), but it is certainly an important and enlightening one in the realm of psychobiology.